1. Field of the Invention
The present invention concerns the field of optical filters consisting of Bragg gratings photo-written in waveguides. The invention concerns more particularly gain-equalising filters.
2. Description of the Prior Art
Gain-equalising filters, also known by the acronym GFF, standing for Gain Flattening Filter, in general consist of Bragg gratings photo-written on portions of waveguides such as optical fibres or planar guides. A waveguide is conventionally composed of an optical core whose function is to transmit and possibly amplify an optical signal, surrounded by an optical cladding whose function is to confine the optical signal in the core. To this end, the refractive index of the core n1 and cladding n2 are such that n1>n2. As is well known, the propagation of an optical signal in a single-mode waveguide breaks down into a fundamental mode guided in the core and secondary modes guided over a certain distance in the optical core/cladding assembly, also referred to as cladding modes.
The core and/or cladding of the guide can be doped so as to be made photosensitive for a Bragg grating writing, for example with germanium (Ge). The gratings conventionally used for gain flattening are slanted gratings, known by the term SBG, standing for Slanted Bragg Grating, or long-period gratings, known by the term LPG, standing for Long Period Grating. Such gratings are non-reflective and are designed to allow coupling of the fundamental mode in the cladding modes. It is also possible to dispense with the optical isolators which are essential when the gain flattening is achieved with reflective gratings such as straight Bragg gratings.
Gain flattening filters are associated with optical amplifiers regularly distributed along transmission lines. Optical amplifiers do not generally provide equal amplification for all wavelengths of the signals transmitted over the various channels of the same transmission line.
In particular, with the development of dense wavelength division multiplexing (DWDM) transmission applications, the disparities in amplification on a given passband have a tendency to be accentuated and the tolerances of the gain flatteners become less and less, that is to say the flattening filter must follow the amplification curve as closely as possible. Thus DWDM applications require the production of narrower and narrower filters exhibiting more and more accentuated contrasts.
The gain flattening profiles are therefore becoming more and more complex and the manufacturing constraints are pushing for minimising the number of writings of gratings per filter.
In the case of slanted gratings (SBGs) for gain flattening applications as presented above, various solutions for writing these gratings can be envisaged for producing a complete flattening filter.
A first known solution consists of writing various SBGs on various waveguides in order to constitute so-called elementary filters each adapted to a portion of the spectral amplification band to be flattened. Several SBGs are then selected, according to the flattening required, and associated with one another in order to form a so-called complex filter. In general, the various SBGs are assembled by welding the various waveguides. Such a technique is however not optimal since the welds on the various elementary filters introduce significant insertion losses.
To mitigate this drawback, it has been proposed to write the various slanted gratings on different portions of the same waveguide. Such a solution is described in the patent application WO 93/24977. The graph in FIG. 1 illustrates the spectrum of the losses of eight uniform SBGs written with an angle of 8° on the same guide portion in order to form a complex filter whose spectral response is the sum of the spectral responses of each SBG. According to the contrast given to each grating, it is thus possible to model the response of the complex filter.
This solution does however require as many writings through as many phase masks as there are elementary filters. The more complex the gain flattening profile, the higher the number of elementary SBG writings necessary for producing the complex flattening filter, which makes the manufacture of such filters more expensive.